1. Field of the Invention
This invention relates to integrated circuits containing logic circuits and embedded Dynamic Random Access Memory (DRAM). More particularly, this invention relates structures of pass transistors within the embedded DRAM such that the processing of the pass transistor is equivalent to that of transistors that form the logic core.
2. Description of the Related Art
Application Specific Integrated Circuits (ASIC) contain sections of circuitry that implement digital logic, provide peripheral circuits to interface to system function, memory, and analog functions. As DRAM has been embedded into an ASIC, the process for constructing the ASIC has become more complex.
Refer now to FIGS. 1a, 1b, and 1c for discussion of the structure of an ASIC containing a logic core and an embedded DRAM array. FIG. 1b illustrates an embedded DRAM cell. The embedded DRAM cell has a pass transistor Mp 105 and a cell capacitor C.sub.c 102. The first plate of the cell capacitor C.sub.c 102 is connected to the drain of the pass transistor Mp 105. The second plate of the cell capacitor is generally connected to the substrate biasing voltage source V.sub.ss. The second plate of the cell capacitor may alternatively be connected to a biasing voltage source that is one half the voltage level of the power supply voltage source V.sub.DD.
The source of the pass transistor Mp 105 is connected to a bit line voltage generator V.sub.BIT. The bit line voltage generator V.sub.BIT generates signals that have the appropriate voltage levels that are used to charge or discharge the cell capacitor C.sub.c 102. Generally the voltage level of the power supply voltage source V.sub.DD is used to represent a logical 1 and the ground reference level represents a logical 0.
The gate of the pass transistor Mp 105 is connected to the word line voltage generator V.sub.WORD. The word line voltage generator V.sub.WORD activates the pass transistor Mp 105 when brought to a voltage level greater than the threshold voltage level Vt of the pass transistor Mp 105. If bit line voltage generator V.sub.BIT and thus the source of the pass transistor Mp 105 is at the voltage level of the power supply voltage source V.sub.DD, the word line voltage generator has to have a voltage level that is from 1.5-2 times the voltage level of the power supply voltage source.
A basic logic circuit is shown in FIG. 1c. The N-channel MOS transistor M1 110 and the P-channel MOS transistor M2 115 are configured as a CMOS inverter. By appropriate placement of additional N-channel and P-channel MOS transistors within the circuit, more complex logic functions can be created. Further, the inverter can be constructed as a driver or receiver within peripheral circuits of the logic core by appropriate design of the transistor parameters and geometry.
The structure of a DRAM cell is shown in FIG. 1a. The pass transistor Mp 105 is formed on the semiconductor substrate 100 by first implanting an N-type material to a lightly doped concentration into the surface of the substrate 100 to form a deep N-well 125. A P-type material is then implanted into the surface of the semiconductor substrate 100 within the area of the deep N-well 125 to form the P-well 130. The N-type material is then diffused to highly doped concentration into the P-well 130 to form the source 135 and drain 140 of the pass transistor M.sub.p 105.
The cell capacitor C.sub.c 120 is then formed by techniques known in the art, such as stacked capacitor or trench capacitor formation.
The transistors M1 115 and M2 110 of the logic circuit are formed concurrently with the pass transistor M.sub.p 105. The N-type material is implanted to the lightly doped concentration to form the N-well 175. The N-type material is implanted to a highly doped concentration to form the source 155 and drain 160 of the N-channel transistor 110.
The P-type material is implanted to a highly doped concentration into the surface of the semiconductor substrate 100 to form the source 180 and the drain 185 of the P-channel MOS transistor 115.
A thin gate oxide is formed on the surface of the semiconductor substrate 100 in the areas 165 and 190 above the channel region between the source 155 and the drain 160 of the N-channel transistor 110, and the source 180 and the drain 185 of the P-channel transistor 115.
Since the voltage level of the word line voltage generator V.sub.WORD is as much as twice the voltage level of the power supply voltage source V.sub.DD, the gate oxide 145 above the channel between the source 135 and the drain 140 of the pass transistor M.sub.p 105 is deposited as a thick gate oxide to prevent excess stress within the thick gate oxide 145. Having multiple thicknesses of the gate oxides 145, 165 and 190 complicates the fabrication process of an ASIC that includes embedded DRAM thus increasing cost. Further, having multiple steps of gate oxide formation causes a higher defect density with the gate oxide.
Typically, the thick oxide is formed to a thickness of from approximately 70 .ANG. to approximately 150 .ANG., while the thin oxide has a thickness of from approximately 30 .ANG. to approximately 70 .ANG. for the logic circuit and the peripheral circuits. Generally the peripheral circuits have a higher operating voltage, and therefore require the thickness of the thin oxide and the thick oxide to be thicker than that of the logic circuit.
U.S. Pat. No. 5,668,035 (Fang et al.) discusses a method for fabricating an ASIC with an embedded memory array and a logic core. The method is described for forming a thin gate oxide for the logic core, while providing a thicker oxide for the memory cells having a boosted word line architecture. The method avoids applying photoresist directly to the gate oxide, and thereby prevents contamination. A first gate oxide is formed on the device areas on the substrate. A first polysilicon layer is deposited and patterned leaving portions over the memory cell areas. The first gate oxide is removed over the logic core areas, and is replaced by a thinner second gate oxide. A second polysilicon layer is deposited and patterned to remain over the logic core areas. The first and second polysilicon layers, having essentially equal thickness, are coated with an insulating layer. The FET gate electrodes for both the logic and memory cell areas are simultaneously patterned from the first and second polysilicon layers to complete the DRAM structure up to and including the gate electrodes.
U.S. Pat. No. 5,702,988 (Liang) describes a method of forming semiconductor logic devices and memory devices on a single semiconductor substrate. The memory devices that may be formed include nonvolatile memory, DRAM and/or SRAM. The method begins by forming triple-well structure of N-well regions, P-well regions, and P-well in N-well regions on a semiconductor substrate. Field isolation regions are then formed. A cell is formed for each memory device in the memory regions. A channel implant is performed in the substrate for each of the logic and memory devices. A gate and gate oxide is formed individually for each of the logic and memory devices. LDD (Lightly Doped Drain) active regions and heavily doped source/drain regions are formed adjacent to each gate. Additional memory structures are formed, such as a DRAM capacitor. An interlevel dielectric and contact openings therein are formed. One or more metal layers are subsequently deposited over the interlevel dielectric layer and in the openings to make contact to the contact regions.
U.S. Pat. No. 5,712,201 (Lee et al.) teaches a semiconductor fabrication process in which both DRAM and logic device structures are integrated on a single silicon chip. The process features combining process steps for both device types, while using only a single level of polysilicon for both a high capacity DRAM cell, as well as for a CMOS logic core. The high capacity DRAM cell is composed of an overlying polysilicon storage gate structure, a thin dielectric layer, and an underlying doped semiconductor region.
U.S. Pat. No. 5,547,893 (Sung) describes a method for simultaneously fabricating memory cells, CMOS devices, and bipolar devices on a semiconductor substrate using a minimum additional number of process steps and process complexity. The method of Sung simultaneously fabricates a CMOS DRAM and a vertical bipolar transistor with a low collector resistance and a N+ polysilicon emitter without using an epitaxy layer.
U.S. Pat. No. 5,600,598 (Skaveland et al.) teaches an embedded DRAM within an ASIC process. Skaveland et al. has a structure for storage of charge that avoids charge leakage from the storage capacitor to the substrate, and eliminates the requirement for a continuous voltage V.sub.BB. The DRAM charge storage structure is comprised of a p-channel access FET in an n.sup.- doped well of a p.sup.- doped substrate, a p.sup.- channel charge storage capacitor, conductive apparatus connecting a plate of the capacitor to a drain of the FET, and apparatus for applying a word line voltage to a gate of the FET.
Skaveland et al. further teaches a DRAM charge storage structure that is comprised of a charge storage capacitor structure connected between a high voltage source V and a source-drain circuit of a storage cell access FET, the gate of the storage cell access FET being connected to a word line. The cell access FET is comprised of first and second p doped regions separated by a channel and contained in an n.sup.- doped region of a p.sup.- doped substrate. The capacitor is comprised of an FET having a gate conductive region insulated from and disposed above the intrinsic n.sup.- doped channel region. The gate conductive region of the capacitor is connected to the second p doped region of the FET spaced from the conductive region. A third p doped region is contained in the n.sup.- doped channel region adjacent the edge of the conductive region. An n doped region is contained in the n.sup.- doped region spaced from the third p-doped region. A high voltage V.sub.pp is applied to the n doped region and to the third p doped region. The bit line is connected to the first p doped region. Voltage is applied from the word line to a gate of the cell access FET that is boosted from the word line voltage V.sub.dd.
U.S. Pat. No. 5,214,603 (Dhong et al.) teaches a folded bit line DRAM cell that includes a trench capacitor and a planar-configured access transistor. The access transistor is stacked over the capacitor and has a first terminal connected to the capacitor. Dhong et al. further describes a folded bit line DRAM cell that includes a vertically oriented access transistor having one terminal formed on the upper extent of a contact to the trench capacitor, to provide optimum electrical connection to between the access transistor and the contact of the trench capacitor.
U.S. Pat. No. 5,436,477 (Hashizume et al.) describes a DRAM memory cell where the transfer gate transistors are formed on a main surface of a semiconductor substrate. The transfer gate transistors have impurity regions for serving as source/drain regions. A first interlayer insulating film having a substantially flat upper surface is formed to cover the transfer gate transistors. The first interlayer insulating film is provided with contact holes reaching the impurity regions. Plugs are formed in the contact holes. Capacitors are only formed on the flat upper surface of the first interlayer insulating film. Lower electrodes of the capacitors and the plugs are electrically connected with each other through barrier layers.
U.S. Pat. No. 5,606,189 (Adan) describes a floating electrode capacitor (FEC) DRAM that occupies a small area on the substrate and which nevertheless has a great capacitance. Adan provides a dynamic RAM comprising (a) a pair of MOS transistors formed side by side on a substrate, (b) a trench formed in the substrate between Source/Drain active regions A and B adjacent to each other at one end of each of the MOS transistors, and (c) a stack capacitor comprising a first electrode layer connected to the impurity region A, capacitor insulating layer and a second electrode layer connected to the impurity region B the layers being formed one over another and embedded in the trench in the order mentioned.